The present invention is directed to a position sensing mechanism for a fuel transfer system in a nuclear power plant, and more particularly to a position sensing mechanism which avoids submerged electrical components even though the fuel is transferred under water.
A typical pressurized water reactor 10 is illustrated schematically in FIG. 1, and includes a reactor vessel 12 which contains nuclear fuel, a coolant (water) which is heated by the nuclear fuel, and means for monitoring and controlling the nuclear reaction. The reactor vessel 12 is cylindrical, and is provided with a permanent hemispherical bottom and a removable hemispherical lid 14. Hot water is conveyed from and returned to vessel 12 by a reactor coolant system which includes one or more reactor coolant loops 16 (usually 2, 3, or 4 loops, depending upon the power-generating capacity of the reactor, but only one loop 16 is illustrated in FIG. 1). Each loop 16 includes a pipeline to convey hot water from the reactor vessel 12 to a steam generator 18, a pipeline to convey the water from steam generator 18 back to the reactor vessel 12, and a pump 20. The steam generator 18 is essentially a heat exchanger which transfers heat from the reactor coolant system to water received at inlet 22 from a source that is isolated from the reactor coolant system; the resulting steam is conveyed via outlet 24 to a turbine (not illustrated) to generate electricity. During operation of the reactor 10, the water within vessel 12 and the reactor coolant system is maintained at a controlled high pressure by pressurizer 26 to keep it from boiling as it is heated by the nuclear fuel.
Nuclear fuel is supplied to reactor in the form of fuel assemblies 28 which are supported by a core plate 30 within vessel 12. There are a number of such fuel assemblies 28 within vessel 12, although only two are illustrated in the drawing. Each fuel assembly includes a base element and a bundle of fuel rods and tubular guides which are supported on the base element. The fuel rods have cylindrical housings which are filled with pellets of fissionable material enriched with U-235. The tubular guides accommodate measuring instruments and movably mounted control rods of neutron-moderating material. A typical fuel assembly for a pressurized water reactor is about 4.1 meters long, has a square cross section that is about 19.7 centimeters per side, and has a mass of about 585 Kg. A typical four loop reactor might contain about 200 such fuel assemblies.
As a safety measure, reactor 10 is enclosed within a containment building 32. Vessel 12 is disposed in a well 34 which can be flooded with water, and the well communicates via a sealable tunnel 36 with a fuel storage building 38 having pools of water for storing fresh and used fuel. Rails 40 extend through tunnel 36 to support a cart 42 which carries fresh fuel assemblies from building 38 to vessel 12 and which returns spent fuel assemblies from vessel 12 to building 38. Cart 42 includes a pivotably mounted fuel container 44 which accommodates one fuel assembly 28.
After a service life of several years in vessel 12, the U-235 enrichment of a fuel assembly 28 is depleted. During a refueling operation, the reactor 10 is shut down, the pressure within vessel 12 is relieved, well 34 and tunnel 36 are flooded with water, and the lid 14 of vessel 12 is removed. Cart 42 is moved to a location adjacent vessel 12 and container 44 is pivoted from the horizontal position to the vertical or upended position. By remote control a spent fuel assembly 28 is removed from vessel 12 and inserted into the awaiting container 44, which is then pivoted back to the horizontal position. Cart 42 is then moved through tunnel 36 to building 38, where container 44 is again pivoted to the upended position and the spent fuel assembly 28 is transferred by remote control to a spent fuel storage pool (not illustrated) containing water with dissolved boron salts to moderate the neutron flux. A fresh fuel assembly 28 is then loaded into cart 42 for carriage back to vessel 12 and transfer thereto. Such exchanges are repeated until a predetermined number of spent fuel assemblies have been replaced by fresh ones, whereupon lid 14 is re-installed, the water is drained from well3 4, tube 36 is seals, and power generation begins anew. Typically there are several such refueling operation during the time a single fuel assembly 28 resides in vessel 12, since vessel 12 includes fuel assemblies 28 at various stages of depletion.
Cart 42 is driven back and forth along rails 40 by an endless chain (not illustrated) that is powered by a motor (not illustrated) positioned above the level of the water. Sensors (not illustrated) are needed in transfer stations at both the fuel-storage side of tunnel 36 and at the containment-building side to determine when cart 42 has reached the location for a fuel transfer, and thus when to stop the motor. Further sensors are needed on each side to determine when container 44 is in the upended position and when it is in the horizontal position. The sensors must, of course, be highly reliable. Furthermore the environment is a harsh one, since cart 42 is under many feet of water when the sensing operations are needed.